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Area Forecast Discussion

332
FXUS62 KCHS 232332
AFDCHS

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHARLESTON SC
732 PM EDT TUE SEP 23 2014

.SYNOPSIS...
A HIGH PRESSURE WEDGE WILL BUILD INLAND...WHILE A COASTAL TROUGH
DEVELOPS OFFSHORE INTO MID WEEK. THE REGION WILL THEN REMAIN
SITUATED BETWEEN HIGH PRESSURE TO THE NORTH AND A STATIONARY
FRONT TO THE SOUTH AND EAST THROUGH EARLY NEXT WEEK.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
THE PATTERN AT THE SURFACE AND ALOFT IS QUITE ANOMALOUS FOR LATE
SEPTEMBER...AND IN FACT TODAY BEING THE FIRST DAY OF AUTUMN AND IT
SURE FELT LIKE IT FOR MANY LOCALES. WE ONLY HIT 72 AT THE
CHARLESTON AIRPORT...THE COOLEST SINCE MAY 2. A FEW PARTS OF THE
CHARLESTON TRI-COUNTY ARE FAILED TO EVEN GET OUT OF THE 60S!

THE SYNOPTIC PATTERN FEATURES A NEARLY STATIONARY MID/UPPER LOW
THAT IS FILLING OVER THE CAROLINAS AND NORTHERN GA...WHILE A WEDGE
OF SURFACE HIGH PRESSURE CONTINUES TO BUILD INLAND AND A
SHARPENING COASTAL TROUGH DEVELOPS OFFSHORE. A SOLID NORTH/NE FLOW
PREVAILS ON THE PERIPHERY OF THE INLAND WEDGE AND IS RESPONSIBLE
FOR KEEPING SKIES MOSTLY CLOUDY TO OVERCAST AREA WIDE.

DEEP MOISTURE STREAMING UP THE EASTERN AND NORTHERN EDGES OF THE
LOW ALOFT...FAVORABLE MID/UPPER DYNAMICS WITHIN THE RIGHT REAR
QUAD OF THE 250 MB JET AND SPOKES OF VORTICITY CIRCULATING
COUNTERCLOCKWISE AROUND THE CYCLONE ALOFT WILL PROVIDE ADDITIONAL
FORCING FOR ASCENT. WHEN COMBINED WITH CONVERGENCE ALONG THE
COASTAL TROUGH ANCHORED OUT TO THE EAST OF THE GULF STREAM...THIS
WILL SUPPORT DECENT RAIN CHANCES TONIGHT...ESPECIALLY EAST OF
I-95. THUS WE HAVE MAINTAINED THE NOTABLE PRECIPITATION GRADIENT
ACROSS THE AREA...WITH CHANCE TO LIKELY RAINFALL IMPACTING THE
COASTLINE AND CHARLESTON TRI COUNTY REGION...WHILE MORE ISOLATED
ACTIVITY OCCURS INLAND ASSOCIATED WITH THE CORE OF THE MID/UPPER
LOW.

THE COASTAL TROUGH WILL AMPLIFY FURTHER OVERNIGHT AND TIGHTEN THE
GRADIENT BETWEEN THE INLAND WEDGE...SUPPORTING A SURGE IN
NORTH/NE WINDS. CONDITIONS WILL BE BREEZY AT TIMES INLAND AND
DOWNRIGHT WINDY ALONG THE COAST...GUSTING UP TO 25 TO 30 MPH AT
TIMES. THE COMBINATION OF THESE COOL TRAJECTORIES AND SMALL COOL
ADVECTION WILL ALLOW TEMPERATURES TO FALL SEVERAL DEGREES BELOW
SEASONAL NORMALS...RANGING FROM THE UPPER 50S INLAND TO THE MID
60S NEAR THE COAST. THESE VALUES WILL BE THE COOLEST AIR MASS SO
FAR THIS SEASON.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH FRIDAY/...
WEDNESDAY...THE CORE OF THE UPPER LOW WILL REMAIN CENTERED JUST
WEST OF THE FORECAST AREA...WHILE A SURFACE TROUGH WILL PERSIST OVER
OR CLOSE TO THE COASTAL WATERS. THROUGH AT LEAST THE MORNING
HOURS...THE UPPER LOW WILL CONTINUE TO DRAW A N-S PLUME OF DEEP
MOISTURE OVER THE COASTAL TROUGH AND INTO COASTAL COUNTIES
ESPECIALLY ALONG THE SC COAST. FARTHER SOUTH/WEST THE UPPER LOW...A
MID-LEVEL DRY SLOT WILL PERSIST CLOSER TO THE UPPER LOW...AND THIS
SHOULD GREATLY LIMIT THE COVERAGE/PERSISTENCE OF ANY SHOWERS.
THUS...MORNING POPS ARE GRADUATED FROM LIKELY ALONG THE SOUTH
CAROLINA COAST...WHERE LOCALLY HEAVY RAIN IS POSSIBLE...TO SLIGHT
CHANCE INLAND. DURING THE AFTERNOON...23/12Z GUIDANCE SUGGESTS THAT
THE UPPER LOW WILL GRADUALLY UNRAVEL/WEAKEN...AND FORCING FOR
ASCENT/ASSOCIATED POPS SHOULD DIMINISH ALONG THE SC COAST.
OTHERWISE...HIGH TEMPS ARE EXPECTED TO REMAIN BELOW NORMAL...RANGING
FROM THE LOWER/MID 70S NORTH/NEAR THE SC COAST TO UPPER 70S/NEAR 80F
INLAND.

WEDNESDAY NIGHT THROUGH FRIDAY...23/12Z GUIDANCE AGREES THAT THE
UPPER LOW WILL CONTINUE TO DISSIPATE WEST OF THE REGION IN FAVOR OF
AN OFFSHORE UPPER RIDGE EXPANDING OVER THE REGION. THE SURFACE
TROUGH WILL LIKELY LINGER OVER OR CLOSE TO THE COASTAL WATERS...AND
MOISTURE WILL REMAIN SUFFICIENT FOR POPS TAPERED FROM CHANCE ALONG
THE COAST TO SLIGHT CHANCE INLAND...HIGHEST DURING THE AFTERNOON.
MEAGER INSTABILITY JUSTIFIES NO MENTION OF THUNDERSTORMS OVER LAND
THROUGH THE PERIOD.

TEMPERATURES SHOULD GRADUALLY MODERATE AND HIGH TEMPS COULD APPROACH
NORMAL VALUES AS EARLY AS FRIDAY.

&&

.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
CONSIDERABLE DISAGREEMENT BETWEEN MODELS IN THE LONG TERM THOUGH IT
LOOKS AS THOUGH THE EXPANSIVE HIGH WILL REMAIN NORTH OF THE AREA
THROUGH SUNDAY. ENOUGH MOISTURE AND LIFT WILL EXIST FOR ISOLATED TO
SCATTERED SHOWERS ACROSS THE AREA. POTENTIAL FOR INCREASING RAIN
CHANCES EARLY NEXT WEEK IF A FRONT LIFTS NORTH OUT OF THE GULF PER
THE 00Z ECMWF. TEMPS RETURN TO NEAR NORMAL FOR LATE SEPT BY THE
WEEKEND.

&&

.AVIATION /00Z WEDNESDAY THROUGH SUNDAY/...
A CLIMATOLOGICAL BASED FORECAST SUGGESTS THAT WITHIN THE COOL-AIR
WEDGE AND A CONTINUED FEED OF NORTH/NE WINDS THAT SINCE WIDESPREAD
LOW CLOUD DECKS ARE ALREADY IN PLACE WE SHOULD REMAIN AT MVFR OR
LOWER THROUGH THE NIGHT AND INTO WEDNESDAY AT BOTH TERMINALS.
WHILE WE ARE FORECASTING SOLID MVFR CONDITIONS...IFR IS CERTAINLY
POSSIBLE AT TIMES...AND THIS IS SHOWN BY A FEW SITES NEAR KCHS AND
KSAV THAT ARE DOWN UNDER 1000 FT OVERCAST THIS EVENING. SPORADIC
LIGHT TO PERHAPS MODERATE RAINS WILL IMPACT KCHS AS A CONTINUED
INFLUX OF MOISTURE STREAMS IN FROM OFF THE ATLANTIC WITHIN A DEEP
SOUTH/SW FLOW ABOVE ABOUT 8-10K FT.

MEANWHILE...A TIGHT GRADIENT BETWEEN INLAND HIGH PRESSURE AND A
PRONOUNCED COASTAL TROUGH WILL SUPPORT GUSTY NORTH/NE WINDS FOR
MUCH OF WEDNESDAY. ALTHOUGH NOT ENOUGH TO WARRANT THE INCLUSION OF
WIND SHEAR GIVEN THE ELEVATED SURFACE WINDS...PILOTS ARE ADVISED
THAT WINDS WILL BE AS HIGH AS 30 OR 35 KT TONIGHT INTO WEDNESDAY
MORNING AROUND 1-2K FT.

EXTENDED AVIATION OUTLOOK...MVFR OR LOWER CIGS POSSIBLE AT TIMES
INTO THE END OF THE WEEK WITH A LINGERING WEDGE PATTERN AND AMPLE
LOW LEVEL MOISTURE.

&&

.MARINE...
TONIGHT...WE HAVE CONTINUED THE PREVIOUS FORECAST WHICH SHOWS
SOLID SMALL CRAFT ADVISORIES IN PLACE ACROSS THE ENTIRE MARINE
COMMUNITY...COURTESY OF A STRONG NORTH/NE PINCHING OF THE GRADIENT
BETWEEN FORMIDABLE INLAND HIGH PRESSURE WEDGE AND A WELL-DEFINED
COASTAL TROUGH EAST OF THE GULF STREAM. THERE IS LITTLE OVERALL
CHANGE IN THIS SCENARIO THROUGH THE NIGHT...ALTHOUGH LAND BREEZE
INFLUENCES WILL ATTEMPT TO HOLD WINDS MORE NORTHERLY CLOSER TO THE
COAST THAN FURTHER AWAY FROM THE SHORELINE. WE HAVE SEEN FREQUENT
GUSTS UP NEAR OR PAST 35 KT OR GALE FORCE WINDS AT BUOY 41004 FOR
SEVERAL HOURS DURING THE LATE AFTERNOON/EVENING...AND WHILE A FEW
GUSTS WITHIN OUR LOCAL WATERS COULD APPROACH GALES...THE MORE
FREQUENT GUSTS WILL BE CAPPED NEAR 30 KT. SEAS WILL BE MAINLY
SHORT-PERIOD AND WIND DRIVEN...AVERAGING 3-5 FT NEAR THE
IMMEDIATE SHORE TO AS MUCH AS 6 OR 7 FT OUT NEAR 20 NM
OFFSHORE...TO 8 OR 9 FT FURTHER OUT. GIVEN THE LOW STRATUS
DECKS...VISIBILITY WILL BECOME SOMEWHAT RESTRICTED AT TIMES...BUT
NOTHING WORSE THAN ABOUT 3 NM.

THROUGH LATE WEEK....STRONG HIGH PRESSURE WILL REMAIN CENTERED TO
THE NORTH...A STATIONARY FRONT WILL REMAIN SOUTH AND EAST OF THE
AREA...AND A COASTAL TROUGH WILL MEANDER OVER OR CLOSE TO THE
WATERS. THIS PATTERN SHOULD SUPPORT A PROLONGED PERIOD OF ELEVATED
NORTHEAST WINDS AND ASSOCIATED ELEVATED SEAS. HOWEVER...23/12Z
GUIDANCE HAS REDUCED WINDS/SEAS DURING THE WEDNESDAY-FRIDAY TIME
FRAME...AND THIS INTRODUCES FORECAST UNCERTAINTY. PERHAPS HIGHEST
WINDS/SEAS WILL PERIODICALLY SURGE INTO THE REGION...AS OPPOSED TO
THE RELATIVELY STEADY STATE CONDITIONS DEPICTED BY THE ONGOING
FORECAST. FOR NOW...SIGNIFICANT CHANGES ARE NOT JUSTIFIED...AND
SMALL CRAFT ADVISORIES WILL CONTINUE INTO WEDNESDAY NIGHT FOR ALL
MARINE ZONES OUTSIDE CHARLESTON HARBOR.

NEXT WEEKEND...HIGH PRESSURE WILL REMAIN TO THE NORTH AND THE
STATIONARY FRONT WILL REMAIN SOUTH/EAST OF THE WATERS. HOWEVER...THE
HIGH PRESSURE COULD EXPAND ACROSS THE REGION...PERHAPS SUPPORTING A
SUBTLE RELAXATION OF THE SURFACE PRESSURE GRADIENT AND ASSOCIATED
WIND/SEAS.

RIP CURRENTS...LOCAL GUIDANCE YIELDED A BORDERLINE LOW/MODERATE RISK
FOR RIP CURRENTS ON AREA BEACHES WEDNESDAY. GIVEN SOME UNCERTAINTY
REGARDING WIND DIRECTION/SPEED...WITHOUT A SIGNIFICANT CHANGE IN THE
ONGOING PATTERN...OPTED FOR A PERSISTENCE FORECAST OF MODERATE
RISK FOR WEDNESDAY. AN ELEVATED RISK FOR RIP CURRENTS WILL LIKELY
CONTINUE THROUGH AT LEAST LATE THIS WEEK...AND PERHAPS EVEN INTO
THE WEEKEND.

&&

.TIDES/COASTAL FLOODING...
A PROLONGED NORTHEAST FLOW WITH MODERATE WIND SPEEDS WILL RESULT IN
ELEVATED TIDE LEVELS THROUGH THE UPCOMING WEEKEND. ALTHOUGH THE
POSITIVE ANOMALY WOULD NEED TO BE A LITTLE MORE THAN 1.0
FEET...THERE IS A MODERATE CHANCE OF THE TIDE REACHING 7 FT MLLW
ALONG THE SOUTH CAROLINA COAST THAT WOULD NECESSITATE COASTAL FLOOD
ADVISORIES AROUND THE TIMES OF HIGH TIDE.

&&

.EQUIPMENT...
THE DOWNTOWN CHARLESTON WEATHER SENSOR AT WATERFRONT PARK /KCXM/
REMAINS OUT OF SERVICE...AND HOURLY OBSERVATIONS AND DAILY CLIMATE
INFORMATION WILL NOT BE AVAILABLE UNTIL FURTHER NOTICE. IT IS
UNKNOWN AT THIS TIME WHEN THE SENSOR WILL BE RETURNED TO SERVICE.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...NONE.
SC...NONE.
MARINE...SMALL CRAFT ADVISORY UNTIL 5 AM EDT WEDNESDAY FOR AMZ330.
     SMALL CRAFT ADVISORY UNTIL MIDNIGHT EDT WEDNESDAY NIGHT FOR
     AMZ350-352-354-374.

&&

$$

NEAR TERM...
SHORT TERM...SPR
LONG TERM...JRL
AVIATION...
MARINE...
TIDES/COASTAL FLOODING...
EQUIPMENT...






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